This project includes a continuing series of theoretical studies and analysis of experiments regarding hormone receptor interactions. The theoretical studies seek to develop appropriate mathematical models for the description of hormone-receptor interactions and response couplings. A variety of model biological systems have been used to develop and test these mathematical models such as opiate binding to its receptors, glucocorticoid binding to receptors, GABA and muscimol binding, and glutamate binding. By development of appropriate mathematical models and computer programs for curve fitting and data analysis we have been able to characterize several of these ligand receptor systems. Special attention has been given to the case of multiple ligands reacting simultaneously with multiple classes of sites and the interaction of divalent ligands with cell surface receptors.